A CPP (Current-Perpendicular-to-Plane)-Tri-layer type magnetic head has been known as a magnetic head used for data read by a magnetic recording/reproducing apparatus such as an HDD (Hard Disk Drive). The CPP-Tri-layer type magnetic head has the following features. First, when there is no external magnetic field from a magnetic recording medium, a hard bias layer makes almost perpendicular the relative magnetization directions of two magnetization free layers which form a magnetoresistive element. Second, the relative magnetization directions of the two magnetization free layers change from almost perpendicular directions to parallel or antiparallel directions by the external magnetic field from the magnetic recording medium. Third, a current perpendicular to the film surface is supplied to the two magnetization free layers to detect the resistance value of the magnetoresistive element, thereby reading data.
At present, signal reproduction of the HDD uses a TMR (Tunneling MagnetoResistive) head in which a current is supplied in a direction perpendicular to the film surface. In the future, miniaturization of a reproducing element is inevitable along with higher recording densities. A magnetoresistive effect element having a low resistance per unit cross sectional area becomes necessary.
For example, it is expected that implementation of a surface recording density of 2 Tb/inch2 requires a reproducing element having an energization cross sectional area of about 20 nm2 . A sheet resistance (RA: energization cross sectional area x resistance) of about 0.3 Ωμm2 or lower is required. In the TMR head, a current flows through a tunnel barrier in principle, so reduction of the resistance has a physical limitation. Under the circumstance, new magnetoresistive effect elements having a high magnetoresistive effect change ratio (MR ratio) at a low resistance have been studied enthusiastically. To increase the recording density of the HDD, the magnetoresistive effect element needs to be downsized.
For example, to implement high-resolution reproduction, the reproducing shield interval needs to be 20 nm or smaller at a surface recording density of 2 Tb/inch2 or higher, and 15 nm or smaller at 3 Tb/inch2. Further, to implement high-track-density reproduction at these recording densities, a magnetization free layer requires a width of about 15 to 20 nm. However, a current TMR head is formed from an antiferromagnetic film (IrMn alloy), a magnetization pinned layer, an intermediate insulating layer, and a magnetization free layer. It is therefore difficult to decrease the total thickness to 15 to 20 nm or smaller and interpose the TMR head between reproducing shields.
To solve this, a so-called Trilayer structure having two magnetization free layers without using an antiferromagnetic film has been examined. This structure is considered to be able to decrease the thickness to almost 10 nm because there is no antiferromagnetic film.